In the production of tin-plated steel for food and beverage cans the steel is plated with tin on an electrolytic tin-plating line. The tin-plate coating is generally melted or "reflowed" and quenched to produce a bright and shiny appearance. As a result of reflowing, a film of tin oxide develops on the surface of the tin-plate. If the growth of this oxide is not stopped or inhibited, it can lead to discoloration when the steel is used for packing certain products. To inhibit oxide growth, the reflowed strip is subjected to a cathodic dichromate treatment in subsequent sequential sections of the electrolytic plating line to passivate the tin-plate surface. After this passivation, a thin film of synthetic oil is electrostatically applied uniformly on the surface before shipment to customers for lacquering with an organic coating. The lacquer may or may not include an adhesion promoter to cause the lacquer to adhere to the dichromate treated surface of the tin-plate. Can manufacturers have experienced sporadic occurrences of poor adhesion of certain epoxy phenolic lacquers despite the inclusion of adhesion promoters in the lacquer. Historically the adhesion problem has been linked to the presence of tin oxide that forms on the surface of the tin-plate and which continues to grow slowly in storage as mentioned above. Varying the solution concentration, temperature, pH, current and current density of the cathodic dichromate treatment as well as the line speed of the strip through the treatment tank has not solved the problem of sporadic poor lacquer adhesion on tin-plate product. It has been further suggested that the adhesion problem could be due to additional tin and chromium oxides that rapidly develop during the lacquer baking cycle. The thermally induced tin oxide is thought to be weak and fragile. However, it has not been possible to fully determine the nature of these oxides and differentiate the thermally induced oxides from the more stable forms. Differences in the formulation of the epoxy phenolic lacquers used by various can manufacturers are also thought to be one cause of the problem. For example, several epoxy phenolic lacquers commonly used for food cans have been found to exhibit differing degrees of sensitivity to variations in the cathodic dichromate treated tin-plate surface. There are many factors that effect the degree of sensitivity of the lacquer to the surface of the tinplate, such as the molecular weight of binders contained in the lacquer, the ratio of different binders in the lacquer, and other factors that appear as numerous as the number of lacquer formulations. To date efforts to solve the adhesion problem have not been successful
U.S. Pat. No. 5,753,779 to Verberg, discloses an aqueous chromium-free coating for treatment, particularly the pretreatment, of metal packaging for the foodstuffs industry, for example tin-plate and aluminum. The coating comprises acrylic acid or a derivative and a crosslinking system comprising at least one hydroxide of a divalent metal and an amine compound. The acrylic acid or derivative and the crosslinking system are used in a ratio of 1.5-4 to 2-4. Preferably the acrylic acid derivative is an acidic resin which comprises 2-8% of the coating solution. The coating solution is applied to a degreased substrate that has been freed of oxide such that the layer has a weight of 25-4000 milligrams per square meter after drying. The coating is safer to use on health grounds than chromium conversion coatings and has comparable properties such as lacquer adhesion.
U.S. Pat. No. 4,104,419 to Tanikawa et al, discloses a method for producing electrolytic tin-plate that is free from discoloration (yellow stain) and having an improved soldering, paint adhesion, resistance to smudges, eye-holeing, and corrosion by sulfides. The electrolytically tin-plated steel is treated with an aqueous solution consisting essentially of 0.1 to 10% of a pyrazole derivative, after which the steel is dried and the tin-plate coating is reflowed and subjected to a conventional chromate treatment. The aqueous solution may also contain 0.1 to 10% of a colloidal substance selected from the group consisting of silica sol and alumina sol.
U.S. Pat. No. 3,138,548 to Ham et al, discloses a method of treating electrolytic tin-plate to improve its lacquer adherent properties. The reference discloses subjecting a tin-plate surface after fusion to treatment by a combination of steps which include: a cathodic treatment in an alkaline electrolyte, a water rinse, a dilute acid dip to bring the surface of the plate to about pH 4-6, and a cathodic chromate treatment in an acidic electrolyte having a pH of about 4-6.
U.S. Pat. No. 3,826,675 to Smith et al, discloses metallic container stock such as tin-plate having a citric acid ester lubricant applied to its surface and which is characterized by excellent wettability and adhesion of organic coatings, especially epoxy resin coating compositions. Preferred citric acid ester lubricants disclosed are triethyl citrate, acetyl triethyl citrate, tributyl citrate, acetyl tributyl citrate, acetyl tri-2-ethylhexyl citrate and mixtures of them. The lubricant is applied to the tin-plated steel strip after flow brightening, quenching and a cathodic dichromate treatment.
U.S. Pat. No. 2,974,091 to Neish, discloses a method of treatment of tin-plate to minimize or eliminate the tendency to "eye-holing" i.e. portions leaving bare spots when lacquered. The steel strip is cleaned, pickled, plated with tin, heated to brighten the deposited tin coating and quenched by a water spray. It is then passed through a stabilizing bath where it is subjected to treatment for the formation of a protective film containing chromium compounds by electrolysis in a solution containing hexavalent chromium. After that the strip is passed through a water rinse and then through a spray or bath of an acidic reducing solution such as a dilute aqueous solution of citric acid or stannous chloride to reduce any free hexavalent chromium to trivalent chromium thereby reducing the tendency to eye-holing on lacquering.
An abstract of Japanese Kokai, JP53084037, assigned to Toyo Ink Manufacturing Company, discloses coating tin-plate with a thin film of metal alcoholate and/or its chelate compound to improve the adherence of ultraviolet curable ink compositions to the tin-plate.
U.S. Pat. No. 4,228,221 to Rohowetz, discloses a method of increasing adhesion of inks and coatings on metal surfaces. The method includes a treatment of the lubricated metal substrate with an organic titanate such as tetrakis (2-ethyl hexyl) titanate and then exposing the surface to ultraviolet light to accelerate the reaction of the titanate with the metal surface.
An abstract of French Patent, FR 2696371, assigned to Ateliers Reunis Caddie SA, discloses applying a varnish underlayer to electrolytic zinc-plated metal to improve the bonding of epoxy resin outer coatings.
U.S. Pat. No. 4,388,158 to Inui et al, discloses steel sheet having an extremely thin tin layer or an extremely thin iron-tin alloy layer having excellent bonding strength after aging in hot water, without deterioration of the bonding strength of organic adhesives. The steel is produced by passing the steel through an acid electrolyte, such as stannous phenolsulfonate or stannous sulfate, to which at least one sulfate selected from the group of sulfates of alkaline metals, ammonium, aluminum, manganese and chromium has been added.
U.S. Pat. No. 4,285,783 to Giza et al, discloses a plastic coating for metal shelving to which an epoxy resin is applied. The metal is plated with a nickel film and a chromium film, which is then treated with an iron phosphate in order to enhance the adhesion of the final outer layer of plastic.
U.S. Pat. No. 3,915,812 to Yamagishi et al, discloses a method for the cathodic treatment of steel strip prior to electrolytic tin-plating to enhance its corrosion resistance. The treating bath comprises an aqueous solution containing from 1 to 10 g/l of caustic soda or sodium orthosilicate and from 5 to 50 g/l of one or more of water soluble oxyacids such as citric acid, gluconic acid, water soluble salts of the oxyacids, a chelating agent having a chelating capability in an alkaline region or an aqueous solution containing from 1 to 10 g/l of sulfuric acid and from 5 to 50 g/l of glucocinic acid, water soluble salts of gluconic acid or a mixture thereof.
U.S. Pat. No. 4,402,747 to Bird et al, discloses a rust inhibiting coating for steel comprising dioctyl sebacate, sodium benzoate and certain sarcosine surfactants.
A literature paper by Barry Arkles entitled "Tailoring Surfaces with Silanes" published by CHEMTEC, December 1977, pages 766-778 discloses organosilanes for altering the wetting or adhesion characteristics of substrates. The reference teaches that organosilanes include R.sub.n nonhydrolyzable organic radical(s) and X.sub.4-n hydrolyzable group(s). The nonhydrolyzble group(s) typically are said to be halogen, alkoxy, acyloxy, or amine. Following hydrolysis, a reactive silanol group is formed, which can condense with other silanol groups, for example, those on the surface of siliceous fillers to form siloxane linkages. Stable condensation products are also formed with other oxides such as those of aluminum, zirconium, tin, titanium and nickel. Less stable bonds are said to be formed with oxides of boron, iron, and carbon. The reference discloses that one can form covalent bonds between an organic moiety and a substrate through an intermediary functional silane, called a coupling agent. Coupling agents are said to be used in enzyme immobilization, solid state synthesis, as antimicrobial agents and dye-fixing agents in textiles, organic surface coatings, and polymer composites and bulk interfaces. The reference states in most cases the substrate is pretreated with the silane-coupling agent, but integral blending accomplished by simple mixing of all the reactants at once, particularly for coatings and composites, is frequently used. The reference does not teach or suggest that the addition of organosilanes to the surface of electrolytic tin-plated steel before, during or after chromate treatment would eliminate poor adhesion of lacquers containing adhesion promoters subsequently applied to the surface.
Various other miscellaneous references are as follows: U.S. Pat. Nos. 3,245,577: 3,227,637; 3,322,656; 4,487,663; 4,812,365; 5,084,358; and 5,248,405.